(1) Field of the Invention
The present invention relates to a near infrared absorption composition which absorbs the near infrared light (800 to 1,000 nm) emitted from, for example, various display devices and is therefore used suitably as a near infrared absorption filter capable of preventing the malfunctioning of electronic appliances placed near the filter. More particularly, the present invention relates to a near infrared absorption i composition which has a high transmittance for visible light and high absorbance for near infrared light and is therefore used suitably as a near infrared absorption filter particularly for a plasma display panel.
(2) Description of the Prior Art
In recent years, large displays of various types have been developed and commercialized. A plasma display is one of such displays. A plasma display generates a near infrared light during the plasma discharge, as easily understood from the operational mechanism; and since remote control systems of house-hold electronic appliances (such as a TV, air-conditioner, video tape recorder, etc.) use the same or similar wavelength of the near infrared region, a plasma display may cause false functioning of those appliances placed nearby.
Hence, it was proposed to utilize a near infrared absorption filter which can absorb and shield a near infrared light, particularly a region of 800 to 1,000 nm. As such a near infrared absorption filter, there can be mentioned, for example, (1) a filter made of a phosphate glass containing bivalent copper ion, (2) a filter obtained by forming, on a substrate (erg. a glass), a thin layer of metal (e.g. silver) by vapor deposition, sputtering, ion-plating or any other methods, and (3) a filter obtained by adding, to a resin, a dye capable of absorbing a light of the near infrared region.
The above near infrared absorption filters, however, have respective problems. The filter (1) is hygroscopic and employs a complicated production process. The filter (2) has a problem of the reflecting not only the light of near infrared region but of the visible region although its amount is smaller than that in the near infrared region. And also it lowers its transmittance when it is too thick, and has a high production cost as well. In the filters (1) and (2), a glass is used; therefore, the filters are heavy, crack easily, and are difficult to mold.
In contrast, the filter (3) obtained by adding, to a resin, a dye capable of absorbing a light of the near infrared region has various advantages such as light weight as compared with glass-made filters and easy production.
As the dye capable of absorbing a light of the near infrared region, there are known cyanine dyes and phthalocyanine dyes. The cyanine dyes are low in light-fastness and unsuitable for use particularly in plasma display panels. The phthalocyanine dyes show absorption for a light region of 600 to 800 nm but are unable to effectively shield a light region of 800 to 1,000 nm (this shielding for 800 to 1,000 nm is necessary particularly for plasma display panels). Moreover, both the cyanine dyes and the phthalocyanine dyes show high absorbance for the visible light region and are unsuited as a dye for a plasma display panel which requires a high transmittance for visible light.
In contrast, dithiol-metal complexes as a near infrared-absorbing dye are generally superior in heat resistance, are low in visible light absorption as compared with other dyes, and are convenient for use in displays; and it was proposed to use such complexes in displays (see JP-A-9-230134, JP-A-10-62620, JP-A-10-156991 and JP-A-10-157023).
These dithiol-metal complexes, however, have problems when used in displays. The dithiol-metal complexes having the structures described in JP-A-10-156991 and JP-A-10-157023 are unable to exhibit, when used per se, the properties required for use in plasma displays, and must be used in combination with a diimmonium type dye which is an other near infrared-absorbing dye.
The dithiol-metal complexes having the structures described in JP-A-9-230134 and JP-A-10-62620 have no long-term stability and require joint use of an ultraviolet-absorbing layer. The dithiol-metal complex used in the examples of JP-A-9-230134 does not show uniform absorption in a wave-length region of 800 to 1,000 nm and does not efficiently shield the region.